6. Examples of Spatial Parsing using Visual Grammars
6.1 Feature Structures (Directed Acyclic Graph Notation Used by Linguists)
Visual language description: Feature Structures are a notation for directed acyclic graphs of a certain type used by linguists. The Feature Structure (FS) in Figure 6 encodes various kinds of syntactic information (barlevel, category, next) and semantic information ( functor) for a natural language expression (further details on the meaning of FSs and their function may be found in [Shieber85]). Looking at the grammar for the FS notation, we see that compared to the grammar for bar charts it uses enclosures and is deeply recursive. The basic FS ( *f-s*) is a pair of brackets surrounding an atribute-value pair list (*a-v-p-list*). An attribute-value pair list is either an attribute-value pair (*a-v-pair*) on top of an attribute-value pair list, or simply one attribute-value pair.
Action taken based on the parse: The parse tree from the FS notation is easily processed to produce an isomorphic LISP s-expression which is then used as input to a function that creates an internal structure of the kind employed by the PATR natural language understanding system at SRI [Shieber85].
Figure 6. Grammar, parse and interpretation for an expression in Feature Structure notation.